#include <bits/stdc++.h>
using namespace std;

using Real = long double;
using llt = long long;

struct Point{
    llt x;
    llt y;
};

Point Ma, Wu;

void work(){
    cin >> Ma.x >> Ma.y >> Wu.x >> Wu.y;
    Wu.y = -Wu.y;
    Real x = Ma.x - Wu.x;
    Real y = Ma.y - Wu.y;
    llt t = sqrt(x * x + y * y);
    cout << t << "\n";
}

int main(){
#ifndef ONLINE_JUDGE
    freopen("z.txt", "r", stdin);
#endif
    ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
    int nofkase = 1;
    // cin >> nofkase;
    while(nofkase--) work();
    return 0;
}
